After the verdict, Leeds Teaching Hospitals NHS Trust apologised to the victims' families for Norris's "disturbing" crimes and described him as an "extremely dangerous criminal".

Medical director, Dr Hugo Mascie-Taylor, said changes implemented at the trust's hospitals included the wider use of CCTV, card entry to parts of the hospital and a permanent police presence on site.

But he said the trust believed none of those actions would have prevented Norris from killing.

West Yorkshire Police said that during their investigation more than 7,000 statements had been taken and more than 3,000 exhibits seized.

Speaking outside the court, Det Ch Supt Chris Gregg said: "What has shone through during this investigation and the trial is the absolute professionalism of the medical and nursing staff and the hospitals we've been working in.

"That's with one clear exception - Colin Norris.

"Whilst other people at those hospitals were duly caring for the patients, Colin Norris was doing the opposite.

"He was looking for opportunities to poison them by insulin."

Norris, who was convicted on a majority verdict, will be sentenced on Tuesday.